1. Field of the Invention
The present invention relates to a zero value-detecting circuit for detecting zero values in a 1-bit digital signal, known as DSD (Direct Stream Digital) data or the like recorded on a SACD (Super Audio Compact Disc), i.e., a 1-bit digital signal obtained by encoding an analog signal by delta sigma modulation.
2. Description of the Related Art
In SACD and the like, an analog signal is encoded into a 1-bit digital signal consisting of a data stream of binary values, i.e., 1 and 0 values, that are continuous in the direction of the time axis, and is recorded. Such a 1-bit digital signal is demodulated into an analog signal by a 1-bit DAC (Digital to Analog Converter). As even if the original analog signal contains a stream of zero values, the converted 1-bit digital signal, because of the characteristics of delta sigma modulation, unlike PCM (Pulse Code Modulation) data, does not has a stream of zero values, but has an idle pattern (e.g., 10101010, 10010110, or the like) intrinsic to a delta sigma modulator which is used for sampling. This causes noises in the demodulated analog signal. One procedure for preventing this noise consists of detecting zero values in the analog signal and muting them if zero values are found. For example, Japanese Patent Laid-Open No. 335956/1998 discloses a technique for demodulating a 1-bit digital signal into an analog signal by a low-pass filter, detecting zero values in the analog signal, and performing muting.
The technique disclosed in the above-cited Japanese Laid-Open Publication requires a low-pass filter to demodulate a digital signal into an analog signal for zero value detection, which increases the scale of the analog circuit and causes problems such as accuracy compensation and others.
The idle pattern indicating zero values in the analog signal is intrinsic to a delta sigma modulator. That is, the pattern differs among individual recording media such as SACDs. If an idle pattern is simply detected by pattern matching in the stage of a digital signal to detect zero values, the detection means is inevitably specialized for a specific medium, which has a disadvantage of lack of flexibility.
Furthermore, if the low-pass filter is replaced by a digital filter, the problem that the circuit scale is increased cannot be circumvented.